My iMac just displays a grey screen with a little flashing icon?

I recently won an old iMac from my school (it's one of the ones with the hard drive attached to the back) and i set it up but when i turned it on i heard the startup noise then it went to this grey screen and this little icon appeared and alternately changed back and forth between a ?. What di I do and i dont have the mac os x thing or watever and if i get the application will it read it off my usb or do i need a disc and if i can use a sub how do i bring up the usb screen.

Thx for answering
 
Back
Top